A specific type of display device is a video projector, which takes a video signal and projects a corresponding image on a screen or other flat surface using a lens and an illumination source. Known projection systems intended for use with television or computer displays sometimes use spatial light modulators (SLM), such as a micromirror device, a liquid crystal display (LCD), or a liquid crystal on silicon (LCoS) display. A micro mirror array is a type of spatial light modulator (SLM) that includes an array of cells, each of which includes a mirror plate that can tilt about an axis and, furthermore, circuitry for generating electrostatic forces that can tilt the micro mirror plate. In a digital mode of operation, for example, the mirror plate can be tilted to stop at two positions. In an “on” position, the micro mirror reflects incident light toward a display surface to form an image pixel in an image display. In an “off” position, the micro mirror directs the incident light away from the image display.
Digital control signals are used to deflect the micro-mirrors of a micro-mirror display, as well to control the display elements of other displays, such as plasma and LCoS. These digital control signals operate in two states: an “on” state where the light is directed onto the viewing area; and an “off” state where the light is kept away from the viewing area. This has the effect that each pixel can be directed to be instantaneously displayed as black or white. Grey-scale can be provided by time multiplexing, that is, displaying during only a fraction of the time available. The percentage of time the device places the light in the “on” state versus in the “off” state determines the perceived brightness level of the pixel display—between black (all off) and white (all on). The number of possible light levels of a pixel between black and white during a given modulation time period is a function of the time period for display of the pixel, divided by the shortest modulation increment.
One example of a display system is a red-green-blue (RGB), field-sequential, light-emitting-diode-based (LED-based) micro-mirror display with a 60 Hz video source. At 60 Hz, the display is refreshed or changed each 1/60 second, or every 16.67 ms. As these RGB systems have three LEDs, one red (R), one green (G), and one blue (B), the R, G, and B fields are displayed sequentially, hence the name “field-sequential.” The percentage of time allocated for each of the red, green, and blue LEDs is a function of many variables including LED efficiency and user preference. If each field is on for about ⅓ of the time, the time available for refreshing each field would be one third of the refresh rate, or ⅓ *16.67 ms, which equals 5.55 ms, which is about 5500 μs.
The details of one or more embodiments of the present disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the subject matter of the present disclosure will be apparent from the description and drawings, and from the claims.
It can also be desirable to display color images. Primary colors (sometimes called “base colors”), can include red, green, and blue. Combinations of these colors provide a color gamut recognizable by a human brain.
One technique for providing color images is to provide multiple SLMs, e.g., one for each primary RGB color. Each SLM is illuminated by a different color source, and a different set of control signals are directed to each SLM to control the individual pixels for each of the RGB colors. The three modulated color light beams are superimposed by an optical projection system to form a full-color image. While this solution generally achieves the goal of presenting a color image without substantial image artifacts or defects, it has the drawback of being relatively expensive, particularly in comparison with systems using only a single SLM. This solution also has the problem that it can be optically complex, both with respect to timing precision of the control signals and with respect to precise superimposing of the color light beams.
Another technique for providing color images is to use a single SLM but to time-multiplex, during a video frame, each RGB color in the projected light beam. Thus, in this system, each color is displayed in turn: a red portion of the image, a green portion of the image, and a blue portion of the image. A system to time-multiplex a light beam into different colors can include red, green, and blue light sources, such as produced by light emitting diodes (LEDs) or lasers. These light sources may be switched between “on” states and “off” states in order to produce a succession of red, green, and blue light sources in synchronization with signals sent to the SLM. When LEDs are used as an illumination source, a drive current or drive voltage input to the LEDs can directly affect the brightness of the display. These light sources may be switched between an “on” state and an “off” state in order to produce a succession of red, green, and blue light sources in synchronization with signals sent to the SLM. Another approach is to use a white light source and a color wheel. The color wheel is rotated by a motor, and a white light beam emitted from the white light source is sequentially filtered by the red, green, and blue filters in the color wheel to produce a sequence of red, green, and blue light in the beam.
Variants of these techniques can include time-multiplexing each color twice or more in a single original video display frame, or multiplexing each color with an additional luminance value (Y). While this technique generally achieves the goal of presenting a color image and can be less expensive than a system with three SLMs, it can also be subject to several disadvantages.
First, displays in which the multiple colors are presented to a single SLM might use a color sequencing to assure that at any specific time, only one red, green, or blue color is being presented to the SLM for display. In some instances, a color flicker may result and may be an annoyance to a human viewer. Second, time-division of each frame into three colors (or more) allows less time for those colors to be presented, with the effect that brightness may be significantly reduced (in comparison to a three SLM system). Third, a luminance signal (Y) might be added, e.g., by adding a monochromatic component, to increase general brightness of the image as perceived by the eye and brain of the observer, but this decreases the saturation of the image. Fourth, if an observer's eye is drawn across the display (either by a moving image on the display or a moving object near the display), the edges of at least some objects can appear with substantial color fringes. For example, a solid round object presented with a second object moving across it can present crescent-shaped color fringes on either side, with each crescent taking on a different color. The effect can be relatively annoying or disturbing, and can result in eye muscle fatigue. Fifth, a color gamut of a display device using LED light sources is very wide and is typically different from the color gamut of conventional devices such as cathode ray tube (CRT) or LCD devices that use conventional phosphors. For example, pure green and red color presentations may be different for an LED display device as compared to a CRT or LCD device (See FIGS. 3c and 3D for NTSC or SMPTE industry standards). So, it may be desirable to implement accurate and flexible color matching technology.